Various processes and catalysts exist for the homopolymerization or copolymerization of olefins. For many applications, it is desirable for a polyolefin to have a high weight average molecular weight while having a relatively narrow molecular weight distribution. A high weight average molecular weight, when accompanied by a narrow molecular weight distribution, provides a polyolefin with high strength properties.
Traditional Ziegler-Natta catalysts systems—a transition metal compound co-catalyzed by an aluminum alkyl—are typically capable of producing polyolefins having a high molecular weight, but with a broad molecular weight distribution. Many of these systems are also capable of producing high melting isotactic polypropylene.
More recently a catalyst system has been developed wherein the transition metal compound has two or more cyclopentadienyl ring ligands—such transition metal compound being referred to herein as a “metallocene—which catalyzes the production of olefin monomers to polyolefins. Accordingly, titanocenes, zirconocenes and haffocenes, have been utilized as the transition metal component in such “metallocene” containing catalyst system for the production of polyolefins and ethylene-alpha-olefin copolymers.
Catalysts that produce isotactic polyolefins are disclosed in U.S. Pat. No. 4,794,096. This patent discloses a chiral, stereorigid metallocene catalyst which is activated by an alumoxane cocatalyst which is reported to polymerize olefins to isotactic polyolefin forms. Alumoxane co-catalyzed metallocene structures which have been reported to polymerize alpha-olefins stereoregularly include the ethylene bridged bis-indenyl and bis-tetrahydroindenyl titanium and zirconium (IV) catalyst. Such catalyst systems were synthesized and studied in Wild et al., J. Organomet. Chem. 232, 233-47 (1982), and were later reported by Ewen and Kaminsky to polymerize alpha-olefins stereoregularly. Further reported in West German Off DE 3443087A1 (1986), but without giving experimental verification, is that the bridge length of such stereorigid metallocenes can vary from a C1 to C4 hydrocarbon and the metallocene rings can be simple or bi-cyclic but must be asymmetric. When substituted or unsubstituted indenyl or tetrahydroindenyl based, these metallocenes are bridged in the “1-position” of the (hydro)indenyl ring, and are of C2 symmetry. It is thought that it is the C2 symmetric structure (also referred to as the d/l-enantiomers or racemic complexes) that produces isotactic poly-alpha-olefins. An alternate form is the Cs symmetric or meso form that is thought to produce atactic poly-alpha-olefins.

The use of metallocene compositions in olefin polymerization is known. Some metallocenes containing substituted, bridged indenyl derivatives are noted for their ability to produce isotactic propylene polymers having high isotacticity and narrow molecular weight distribution. Considerable effort has been made toward obtaining metallocene produced propylene polymers having ever—higher molecular weight and melting point, while maintaining suitable catalyst activity. Researchers are exploring the relationship between the way in which a metallocene is substituted, and the molecular structure of the resulting polymer. For the substituted, bridged indenyl type metallocenes, it is thought that the type and arrangement of substituents on the indenyl groups, as well as the type of bridge connecting the indenyl groups, determines such polymer attributes as molecular weight and melting point. Unfortunately, it is impossible at this time to accurately correlate specific substitution patterns with specific polymer attributes.
For example, U.S. Pat. No. 5,840,644 describes certain metallocenes containing aryl-substituted indenyl derivatives as ligands, which are said to provide propylene polymers having high isotacticity, narrow molecular weight distribution and very high molecular weight.
Likewise, U.S. Pat. No. 5,936,053 describes certain metallocene compounds said to be useful for producing high molecular weight propylene polymers. These metallocenes have a specific hydrocarbon substituent at the 2 position and an unsubstituted aryl substituent at the 4 position, on each indenyl group of the metallocene compound.
While metallocenes of this type have their benefits, one of the disadvantages is found in the synthesis of such materials. While the rac isomer is considered more desirable, most common synthetic schemes produce a mixture of rac and meso isomers that can be difficult to separate.
More recently, substituted and unsubstituted indenyl based metallocenes with a bridge in the 4-position have been reported. Some of these catalyst systems have been shown to produce isotactic polypropylene. EP 693502 and U.S. Pat. No. 5,594,081 discloses metallocenes with a 1,2-ethylene bridge or dimethylsilylene bridge bridging the 4-positions of substituted indenyl rings. Some catalyst systems based on the dimethylsilylene bridged complexes were shown to produce isotactic polypropylene.
WO 96/38458, EP 846122, and U.S. Pat. No. 6,369,254 disclose metallocenes with a 1,2-ethylene bridge bridging the 4-positions of substituted indenyl rings. Both rac and meso metallocene isomers are produced. A catalyst system based on the rac isomer was shown to produce isotactic polypropylene, and ethylene propylene copolymers containing propylene crystallinity.
WO 99/26985 and EP 1034190 disclose metallocenes with a 1,2-ethylene bridge bridging the 4-positions of substituted indenyl rings. Only ethylene alone or in combination with norbornene were polymerized.
Additional references of interest include: WO 96/04317; JP 11171925 (JP1999171925); JP 11060588 (JP1999060588); JP 11001508 (JP1999001508); and JP 08301914 (JP1996301914).
The following papers report metallocenes with a dimethylsilylene bridge, methylene bridge, 1,2-ethylene bridge, or 1,3-propylene bridge bridging the 4-position of substituted indenyl rings: 1) Studies in Surface Science and Catalysis 1999, 121 (Science and Technology in Catalysis 1998), 473-476; 2) J. Am. Chem. Soc. 1998, 120(38), 9945; and 3) Organometallics 1998, 17, 3900. In these papers, only the racemic 1,2-ethylene and dimethylsilylene bridged metallocenes were reported to produce isotactic polypropylene. The 1,3-propylene bridged metallocene based catalyst system (a mixture of rac and meso isomers) showed no polymerization activity towards propylene. The methylene bridged complex was reported to form only a racemic complex, however, was also reported to have poor solubility and poor stability in solution. No polymerization data was reported using this complex.
Other references of interest include WO 03/00744 and JP 11 171925 (published Jun. 29, 1999).
In view of the difficulty and practical limitations in the synthesis of bridged metallocene complexes necessary for the production of an activated metallocene catalyst system capable of producing crystalline or non-crystalline poly-alpha-olefins, it would be desirable to develop new catalytic processes which produce poly-olefins of high molecular weight and relatively narrow molecular weight distributions, and/or produce poly-alpha-olefins of high crystallinity, high molecular weight distribution and relatively narrow molecular weight distributions. Additionally, it would be beneficial to be able to form the pre-catalyst complexes exclusively as the racemic versions when desired, or exclusively as the meso version when desired while maintaining good pre-catalyst solubility and stability.